Permanent magnetic materials

In a permanent magnet a material is used that exhibits magnetism even in the absence of a current-carrying coil. The silicon-iron and nickel-iron alloys are ‘soft’ magnetic materials and possess high permeability & have low hysteresis loss. The converse characteristics are necessary in the ‘hard’ materials that are used for making permanent magnets. In permanent magnets, high remanent flux density & high coercive force, after magnetization to saturation, are often required for the purpose of resisting de-magnetization. The hysteresis loop should be able to embrace the maximum possible area.

The best measure of the merit of a permanent magnet is its maximum energy product (BH)m, that is, the maximum value of the product of the flux density B and the magnetic field strength H along the de-magnetization curve. A gross criterion is the product of coercive force and remanent flux density. The initial most materials used for permanent magnets were tungsten and chromium steel, after this a series of cobalt steels, to give both a high remanent flux density & a high value of (BH)m. Alnico was the first of the aluminium-nickel-iron-alloys that was first discovered, & with the addition of cobalt, titanium and niobium, the Alnico series of magnets was developed, the attributes of which depends on their composition. These materials are very hard & brittle. There are many alloys giving other different compositions & trade names are commercially available easily.

There was significant development when it was found that directional magnetic properties can be induced in alloys of suitable composition if we heat them in a strong magnetic field. This discovery led to the powerful Alcomex and Hycomex series of magnets. By employeeing unique casting techniques to give a grain oriented structure, more good properties are achieved if the field applied during heat treatment is parallel to the columnar crystals in the magnet. The values of coercivity, the remanent flux density & therefore (BH)m are more for these kind of alloys.

The most recent & most powerful permanent magnets discovered are made by powder metallurgy techniques & there techniques are based on an inter-metallic compound of cobalt and samarium. You cannot find them easily. The reason is that these are very expensive plus are only available in a limited range of small sizes.